1. Field of the Invention
The present invention relates to energy-trapped piezoelectric resonator components and, in particular, to an energy-trapped piezoelectric resonator component that suppresses a fundamental wave of thickness longitudinal vibration functioning as a spurious wave.
2. Description of the Related Art
Energy-trapped piezoelectric resonators are in widespread use as oscillators. A variety of vibration modes are selectively used depending upon a target frequency.
Japanese Unexamined Patent Application Publication No. 8-148967 discloses an energy-trapped piezoelectric resonator 101 using the fundamental wave of thickness longitudinal vibration. As shown in a perspective view in FIG. 9, the known energy-trapped piezoelectric resonator 101 includes a piezoelectric substrate 102 having a rectangular planar shape. The piezoelectric substrate 102 is made of a piezoelectric ceramic that is polarized in the direction of thickness thereof. A vibrating electrode 103 having a circular planar shape is disposed on the top surface of the piezoelectric substrate 102, and an extension electrode 104 that extends from the vibrating electrode 103 is disposed on the top surface of the piezoelectric substrate 102. A vibrating electrode is also arranged on the bottom surface of the piezoelectric substrate 102 such that the vibrating electrode is opposed to the vibrating electrode 103. An extension electrode extends from the vibrating electrode.
An alternating current electric field is applied between the vibrating electrode 103 on the top surface and the vibrating electrode on the bottom surface of the piezoelectric substrate 102 in the energy-trapped piezoelectric resonator 101 to excite the thickness longitudinal vibration. Large spurious vibrations are caused by parasitic oscillation between the resonance frequency and the anti-resonance frequency of the thickness longitudinal vibration. To control the spurious vibration, paint films 105 and 106 made of an organic polymer containing a ceramic powder are disposed on at least one of the major surfaces of the energy-trapped piezoelectric resonator 101. The use of the paint films 105 and 106 increases the weight of the energy-trapped piezoelectric resonator 10, thereby controlling the parasitic oscillation and the spurious vibration.
The paint films 105 and 106 provide a damping effect to the parasitic oscillation in the use of the fundamental wave of thickness longitudinal vibration. In this technique, however, the paint films 105 and 106 must be applied, thereby increasing manufacturing steps and leading to a cost increase. When casing substrates are arranged on the top and bottom of the piezoelectric resonator 101, sealing performance deteriorates in the area of the piezoelectric substrate having the paint films 105 and 106 containing the ceramic powder applied thereto. The paint films 105 and 106 cause, due to the thickness thereof, difficulty in implementing a low-profile design in the piezoelectric resonator component. The paint films 105 and 106 cannot be uniformly applied because of the presence of the ceramic powder contained in the paint films 105 and 106, and thus, the paint films 105 and 106 fail to control the spurious wave in a stable manner.
Japanese Unexamined Patent Application Publication No. 2003-87077 discloses an energy-trapped piezoelectric resonator using a third order harmonic wave of the thickness longitudinal vibration. Circular vibrating electrodes are disposed on both major surfaces of a piezoelectric substrate having a rectangular planar shape such that the vibrating electrodes are opposed to each other with the piezoelectric substrate interposed therebetween. A damping material is arranged outside an area of the piezoelectric substrate where the vibrating electrodes are opposed. In the piezoelectric substrate, a thickness of the piezoelectric substrate “t” is set to be greater than a distance “L” between the external edge of the vibrating electrode and the damping material. With the damping effect of the damping material, the piezoelectric substrate excites a third order harmonic wave of the thickness longitudinal vibration while controlling the fundamental wave as a spurious wave.
The damping material is arranged to constrain the fundamental wave of the thickness longitudinal vibration in the piezoelectric resonator component working on the third order harmonic wave of the thickness longitudinal vibration. This known technique also takes advantage of the damping effect of the damping material, increasing manufacturing steps and leading to a cost increase.
Depending on variations in the location of the damping material, the damping effect also varies and stable control of the spurious wave is difficult.